František Hrouda

Magnetic anisotropy of rocks and its application in geology and geophysics

Magnetic anisotropy in sedimentary rocks is controlled by the processes of deposition and compaction, in volcanic rocks by the lava flow and in metamorphic and plutonic rocks by ductile deformation and mimetic crystallization. In massive ore it is due to processes associated with emplacement and consolidation of an ore body as well as to ductile deformation. Hence, it can be used as a tool of structural analysis for almost all rock types. Morcover, it can influence considerably the orientation of the remanent magnetization vector as well as the configuration of a magnetic anomaly over a magnetized body. For these reasons it should be investigated in palaeomagnetism and applied geophysics as well.

Theoretical models of magnetic anisotropy to strain relationship revisited

Abstract Mathematical modelling of the relationships between the low-field magnetic anisotropy and strain are redeveloped, and the calculated data are compared with the degree of anisotropy for various rock types. It is found that the degree of anisotropy for realistic strain magnitudes is unrealistically high in the ‘passive’ model. The ‘ductile’ model, for which the degree of anisotropy depends on the magnetic grain the matrix viscosity ratio, becomes realistic at high viscosity contrasts. In the ‘line/plane’ model, the theoretical degree of anisotropy corresponds well to the natural degree of anisotropy for rocks in which the carrier of magnetic anisotropy is either magnetite or phyllosilicate minerals; by contrast, in the case of haematite and pyrrhotite, the modelled degree of anisotropy is much higher than the natural values. In the ‘viscous’ model, the calculated degree of anisotropy corresponds well to that measured in sedimentary and volcanic rocks. In the models imposing pure shear strain, the natural logarithm of the degree of anisotropy to natural strain relationship can be approximately represented by a straight line, at least for low to intermediate strains; the proportionality constant varies according to the specific model and the specific carrier of the anisotropy of magnetic susceptibility.

The effect of quartz on the magnetic anisotropy of quartzite

РезюмеМонокрuсmaллы квaрцa являюmся ¶rt;uaмaгнumнымu (восnрuuмчuвосmь−14 × 10−6 в сuсmеме СИ) u оченъ мaло aнuзоmроnuынмu (не более1%). Эmо знaчum, чmо восnрuuмчuвосmь основноŭ квaрцевоŭ мaссы в квaрцumaх можеm быmь nрuняma nрaкmuческu uсоmроnноŭ. Из мamемamuческого мо¶rt;елuровaнuя сле¶rt;уеm, чmо nо nрuчuне влuянuя оmрuцamельноŭ u uзоmроnноŭ восnрuuмчuвосmu основноŭ квaрцевоŭ мaссы сmеnень aнuзоmроnносmu квaрцuma выше, чем сmеnень aнuзоmроnносmu феромaгнumноŭ фрaкцuu. Эmо влuянuе нauбольее mог¶rt;a, ког¶rt;a сре¶rt;нaя восnрuuмчuвосmь блuзкa нулю.SummaryThe magnetic susceptibility of quartz single crystals is diamagnetic (−14×10−6 in SI units) and exhibits only very small anisotropy (mostly less than 1%); thus the susceptibility of the quartz matrix in quartzite can be regarded as virtually isotropic. Owing to the influence of the negative and isotropic susceptibility of the quartz matrix, the degree of anisotropy of quartzite, as inferred from model calculations, is higher than that of the ferrimagnetic fraction. This influence is very strong if the mean susceptibility of quartzite is in the vicinity of zero.

The changes in shape of the magnetic susceptibility ellipsoid during progressive metamorphism and deformation

Abstract The magnetic anisotropy of a sequence of rocks ranging from shales and greywackes to gneisses was investigated. The oblate and slightly eccentric magnetic susceptibility ellipsoids in sediments are progressively changed when sediments are subject to metamorphism and ductile deformation. In the initial stages of low-grade metamorphism and ductile deformation, the ellipsoids become more eccentric and triaxial, later they are even more eccentric, but again predominantly oblate. The directions of principal susceptibilities are closely related to the primary-fabric elements in undeformed sediments; during deformation they come to be related to the deformational-fabric elements.

The magnetic fabric relationship between sedimentary and basement nappes in the High Tatra Mountains, N. Slovakia

Abstract In the High Tatra Mountains (N. Slovakia), the magnetic fabrics are deformational in origin in sedimentary rocks of the Križna nappe, in sedimentary rocks covering the underlying crystalline complex, and in granitoids and metamorphic rocks of this complex. The patterns of the principal susceptibilities are similar in all these units; their most important feature is a girdle in magnetic foliation poles oriented NNW-SSE to N-S. In sedimentary formations the magnetic fabric pattern is compatible with that predicted by mathematical modelling to develop during a complex deformation comprising lateral shortening and simple shear, which is characteristic of nappe deformation. Consequently, the magnetic fabric in the High Tatra Mountains probably resulted from deformation associated with the formation and movement of the nappes, during which not only the sedimentary strata, but also the crystalline complex, were thrust over the North European platform.

Mathematical model relationship between the paramagnetic anisotropy and strain in slates

Abstract A simple mathematical model is used to investigate the relationship between the paramagnetic anisotropy and strain in slates. The relationship found is similar to the relationship of whole-rock magnetic anisotropy to strain. In weakly magnetic slates, the anisotropy is probably mostly due to paramagnetic phyllosilicates and the whole-rock relationship reflects the paramagnetic fraction.

Siderite formation in anoxic deep-sea sediments: A synergetic bacteria controlled process with improtant implications in paleomagnetism

Recent work on magnetic properties of limestones has demonstrated that the mineral siderite can be very important in paleomagnetism, for two reasons. First, oxidation of siderite produces secondary (daughter) magnetic minerals (magnetite, maghemite, and hematite), either before, during, or after sampling. These daughter products can completely change the magnetic properties of limestone samples and if unrecognized may be one of the primary reasons why many paleomagnetic studies of limestones, especially Paleozoic limestones, are unsuccessful. Second, siderite in weakly magnetized rocks may indicate the potential for successful paleomagnetic results. Because the presence of siderite indicates that the primary magnetic carriers are still intact, appropriate demagnetization methods should yield successful results. We conclude that microenvironmental conditions in anoxic marine sediments may permit the formation of siderite from iron (II) produced during bacterial dissimilatory iron reduction.

Indices for Numerical Characterization of the Alteration Processes of Magnetic Minerals Taking Place During Investigation of Temperature Variation of Magnetic Susceptibility

The alteration of magnetic minerals taking place during the investigation of the temperature variation of bulk magnetic susceptibility is obvious from different courses of heating and cooling susceptibility vs. temperature curves. A set of indices is introduced to characterize these changes numerically. The A40 alteration index characterizes the change in susceptibility after executing the whole cycle of heating and cooling. The maximum difference between the heating and cooling curves is characterized by the Amax alteration index. The mean or average difference between the heating and cooling curves is characterized by the Am alteration index. The situation whether the heating and cooling curves cross, is characterized by the Acr alteration index. The technique of progressive repeated heating is proposed, together with the above indices, to locate the temperature intervals with weak and strong magnetic mineral changes induced by heating.

Models of magnetic anisotropy variations in sedimentary thrust sheets

Abstract Magnetic anisotropy variation in sedimentary thrust sheets was modelled through the extension of strain models in thrust sheets (Coward and Kim, 1981; Sanderson, 1982), using the empirical relationship between the magnetic anisotropy and strain and respecting the pre-deformational anisotropy of sedimentary rocks. In thrust sheets deformed by simple shear and lateral lengthening the magnetic foliations remain near the bedding, while in thrust sheets deformed by simple shear and lateral shortening the magnetic foliation may deviate strongly from the bedding, even creating a girdle pattern in its poles.

Conversion of the magnetic susceptibility tensor into the orientation tensor in some rocks

Abstract The preferred orientation of minerals in a rock with a structural or sedimentary fabric can be referred to by the orientation tensor. This paper describes the determination of the orientation tensor of the crystallographic c′-axes from the rock magnetic anisotropy and the mineral anisotropy degree, in rocks in which the magnetic anisotropy is dominantly carried by one mineral with uniaxial magnetic anisotropy (e.g. phyllosilicates, pyrrhotite, hematite). The orientation tensor of biotite c′-axes in four samples of the Bites orthogneiss determined in this way is similar to that determined independently through the universal stage measurement of biotite leaves in thin sections and is obtained much more rapidly by the magnetic method.